5 Pulse

The importance of examining the pulse in domes­tic animals is that this procedure, in conjunction with examination of the heart and the superficial veins, and when advisable the circulating blood, enables the clinician to formulate an opinion as to the condition of the circulatory system. By this means prognostication is not infrequently assisted.

In the horse, the pulse is taken at the external maxillary artery, on the medial aspect of the ventral border of the mandible where the vessel is associated with the corresponding vein and the parotid duct, and itself continues as the facial artery (Fig. 20). In certain circumstances it may be taken in other positions: at the transverse facial artery, at a point posterior to the zygomatic process of the frontal bone about midway between the base of the ear and the eye; at the median artery beneath the posterior superficial pectoral muscle, at the upper extremity of the foreleg on its medial aspect, as the artery passes downwards in company with the corresponding vein and median nerve; or at the great metatarsal artery on the lateral aspect of the large meta­tarsal bone in the groove formed by the latter and the lateral small metatarsal bone. In restless horses, the median artery will prove most satis­factory.

In the ox, if the animal is quiet, the pulse may be taken at the facial artery on the lateral aspect of the mandible (Fig. 21) or at the trans­verse facial artery. The median artery is also commonly used (Fig. 22). An alternative site is the middle coccygeal artery, which is palpated on the under aspect of the tail about 10 cm below the level of the anus (Fig. 23). The pulse is readily detected at this site in dairy cattle, and with a little practice can be obtained here in any type of bovine animal, including fat bullocks and very young calves. The advantages claimed for taking the pulse at this site are that the temperature can be taken, and the character and rate of the respiration noted at the same time. Important disadvantages are the small size of the artery, which precludes a proper appreciation of pulse quality as distinct from pulse rate, and the great frequency of faecal contamination of this part.

In the dog, cat, sheep, goat, small pig and young calf, the pulse is taken at the femoral artery, high up in the inguinal region, on the medial aspect of the thigh (Fig. 24). In large pigs, pulse frequency can sometimes be deter­mined at the middle coccygeal artery.

If the pulse cannot be detected because of restiveness, generalized muscle tremors, obesity or any other reason, the heart beats are counted, with the aid of a stethoscope if necessary. This is a somewhat unsatisfactory substitute and it is important to realize that the heart rate may not always accurately reflect the pulse rate; in con­ditions such as extrasystolic arrythmia some heart beats do not produce a pulse wave.

When determining the pulse frequency, a watch with a second hand should be used, and the waves counted for a minimum of 30 seconds, when the number obtained should be multiplied by 2. In many cases it is advisable to continue counting the pulse waves for 1 minute, during which time note may be taken of other features such as rhythm and quality. It is advisable to avoid counting for periods less than the minimum recommended, otherwise, unless great accuracy is achieved, significant over- or undercounting may result, more particularly in animals with a high pulse frequency.

The technique of taking the pulse consists of placing the ball part of one or more fingers on the skin over the selected artery, and applying gentle pressure until the pulse wave can be detected. Where the artery is large and tends to roll away from beneath the transversely placed fingertips, as does the external maxillary artery of the horse and the facial artery in cattle, it may be found of value to place the tips of 2, or even 3, adjacent fingers on the artery parallel to its long axis, so that the vessel is held in the groove between the fingertips; this is readily achieved by advancing the second finger beyond the first and third. Smaller arteries are palpated with the tip or ball of the second or third finger. It is better to avoid the use of the index finger because the skin of the tip of this finger is generally somewhat thicker, and therefore less sensitive, than that of the others. Pressure must be just sufficient to tense the skin and the under­lying artery to make the pulse wave detectable and, in the case of a small artery, gentle pressure only is used, in order to avoid obliteration of the pressure wave. Before concluding the examina­tion of the pulse, it is legitimate gradually to increase the pressure applied to the artery to the point at which the pulse wave disappears, and then reduce the pressure again in the same manner. With experience this procedure will prove useful in assessing certain pulse qualities.

The student is advised to practise pulse taking in the various animal species until detection of the pulse is readily achieved, the variations in frequency and character of the normal pulse are fully appreciated and the influence of physio­logical factors readily recognized, and their sig­nificance correctly interpreted. Only when this stage is reached will it become possible to recog­nize significant variations from the normal pulse in individual cases. In an examination of the pulse the following properties should always be considered: (a) rate, (b) rhythm, and (c) quality.

Physiological Factors Affecting the Pulse Rate in Normal Animals

Species. Table 2 indicates the species range and variation. The rates given are for healthy ani­mals accustomed to being handled; in compar­able animals not accustomed to the interference associated with taking the pulse the rates would be up to 25 % higher. In general, the smaller the species the more frequent the pulse rate, but size is not the only factor responsible for the difference; for example, a pony stallion has a lower pulse rate than a bull of the same age, which might be more than three times the weight of the pony.

Size. Within a species the pulse rate is usually, but by no means invariably, higher in small than in large individuals. A healthy working pony at rest, for example, usually has a pulse rate in the region of 40/minute, whereas a heavy draught horse under similar circumstances has a pulse rate of around 30/minute. The effect of size is not so marked in cattle.

Age. The pulse rate in very young animals is much higher than in adolescent and adult indi­viduals of the same species. The neonatal calf, for example, has a pulse rate in the region of 120/minute compared with the rate of over 80 for yearling cattle and 50-80 for adult animals. It should be noted, however, that the compara­tively high pulse rate usually accepted as being normal for the young (as compared with that of the newly born) animal is not attributable solely to immaturity; the excitement and exertion in­variably associated with the examination of the healthy foal or calf by a strange person produce a much greater physiological effect on the pulse rate than does equivalent handling of the adult horse or dairy cow. Thus, although the pulse frequency of young calves is as quoted 100-120/ minute and that of yearling cattle as being in the vicinity of 80/minute, the resting pulse of calves accustomed to being handled may be as low as 80/minute by the fourth, and 60/minute by the eighth week of life.

Physical condition. Athletic animals, particu­larly if in training, have a less frequent pulse than non-athletic animals of the same species and type. This situation is common in race horses, trotting horses, hunter ponies and certain breeds of dogs, more especially greyhounds. In the types of horses mentioned the dropping of a pulse wave when at rest, due to hypertension, partial atrioventricular block or other cause, is not invariably indicative of serious abnormality.

Sex. In most species the male animal has a slightly lower pulse rate than the female.

Pregnancy. In the later stages of pregnancy, the pulse is more frequent than it is in the same animal when in the non-pregnant state. This is probably an indication of a mild degree of hyper­tension. In cattle during the last 3 months of pregnancy the proportional increase in pulse rate may amount to 15-40%.

Parturition. As parturition becomes imminent, there is a further increase in the pulse frequency.

Lactation. Lactating animals have a higher pulse rate than comparable ones not lactating.

Moreover, the heavier the milk yield the greater the increase in pulse frequency. In high-yielding dairy cattle the pulse rate may be 10% more frequent than in those producing only moderate quantities of milk.

Excitement. This may cause a considerable increase in the pulse frequency, particularly in animals unaccustomed to being handled. Even in dairy cattle the approach of a strange person usually causes an immediate increase of up to 10% in the rate of the pulse. In this type of animal, when healthy, the pulse rate usually returns to the normal range within a few minutes if no further cause of excitement should occur. In readily excited subjects the increased pulse frequency persists throughout the period the animal is exposed to a fright-inducing stimulus, e.g. clinical examination procedures, restraint. This situation is the result of a coordinated, in­creased output of adrenal medullary secretion, and of increased activity of the sympathetic component of the autonomic nervous system bringing about redistribution of the blood supply, with an associated increase in heart rate.

Exercise. Physical exertion increases the pulse rate to an extent which varies according to the severity of the exercise and fitness of the animal. In thoroughbred and standardbred horses, elec­tronic monitoring has shown that pulse rates of up to 200/minute may occur during severe exercise such as racing. In dairy cattle, 10 min­utes of forced exercise may produce an increase of over 60% in the frequency. After 30 minutes rest the pulse rate, in such animals, may still be 10% above normal, and may not return to its original resting rate for as long as 90 minutes after the exercise has terminated. In athletic animals such as racehorses, riding horses or greyhound dogs, following exercise, the pulse rate usually returns to normal within 10-15 minutes.

Posture. Except in the horse, which can rest in the standing position, the pulse rate is appre­ciably less frequent when the animal is lying down than when it is on its feet. In cattle the rate of the pulse may be reduced by as much as 10% during a period of recumbency.

Ingestion of food. Eating a large quantity of food will cause a very considerable increase in the frequency of the pulse. In dairy cattle the increase in rate may be as great as 66%; a pulse rate of over 100/minute is not unusual towards the end of a large meal. When ingestion of the food is completed, the pulse rate decreases fairly rapidly, but does not return to its preprandial frequency for some time; an hour after eating it may still be up to 10% higher than it was immediately prior to the meal.

Rumination. This action is recognized to cause an increase of up to 3 % in the pulse rate.

Environmental temperature. Exposure to either very high or very low temperatures produces an increase in the rate of the pulse. In the former situation, the increase is due to the rise in blood temperature acting directly on the heart, which is also affected indirectly by the fall in blood pressure resulting from peripheral vasodilatation. In the latter case increased amounts of adrenaline and noradrenaline are secreted, for calorigenic purposes, and which along with other endocrine secretions cause an increase in heart rate.

An increase in the rate of the pulse occurs in all painful conditions, in many diseases not primarily involving the cardiovascular system, and is one of the characteristic features of the fever syndrome.

Pulse Rhythm

The rhythm of the pulse is assessed by appre­ciation of the time intervals between the peaks of a series of successive pulse waves, and the temporal sequence may be regular or irregular. With the exception of sinus arrhythmia, which is a characteristic feature of pulse rhythm in healthy dogs and more rarely in other animals, and is related to the respiratory cycle, rhythmic irregularities may be regular or irregular in their time relationships. Regular intermittence in the pulse (Fig. 25b) is occasionally observed in ani­mals customarily subjected to severe physical exertion; the intermittence occurs with constant periodicity and it indicates a mild degree of heart block. In such cases the pulse irregularity often disappears with exercise. Irregular inter­mittence occurs without any cyclical pattern being obvious, and is usually associated with irregularities in pulse amplitude due to variations in the stroke volume of the heart. The common­est causes of irregular intermittence of the pulse are second degree heart block, ventricular extra-systoles and auricular fibrillation. With exercise, pulse irregularities of this type usually become accentuated.

Pulse Quality

The quality of the pulse depends mainly upon the amplitude of the pressure waves which may vary because changes in the rate of diastolic filling influence the stroke volume of the heart. Rhythmic irregularities in the pulse are invariably associated with some degree of fluctuation in pulse amplitude. The amplitude or quality of the pulse is assessed by noting the degree of digital pressure required to obliterate the pulse wave in the artery. This is the only practical method readily available to the veterinary cli­nician whereby blood pressure variations and cardiac output can be assessed.

Changes in the normal qualities of the pulse may be caused by structural or functional disease of the heart or by abnormalities of the blood vessels (arterial thrombosis and embolism, vasomotor disturbance, certain types of passive venous congestion) which only have a local or indirect influence on the pulse. Such changes may affect any one or all of the three properties mentioned.

The normal pulse wave (Fig. 25a) is regular in sequence, amplitude and strength, and the artery is well filled, the wall being distended and exhibiting a degree of tone which is readily appreciated by digital palpation. In the wake of the pressure wave it should still be possible to detect residual tone in the wall of the artery. It should be noted that in some species, particu­larly horses and dogs, the degree of distension of the artery at the acme of the pulse wave is proportionately greater than in other species, such as cattle and sheep. This interspecies dif­ference is attributable to variations in the activity of the vasomotor mechanism, and in the amount of elastic tissue in the walls of the arteries, in­fluencing the distensibility and tone of the vessels. It is important to appreciate too that irregularities in the rhythm of the pulse, in the absence of clinically demonstrable disease, are by no means uncommon in the dog, goat, sheep and horse.